Ultra low carbon steel sheets, by virtue of excellent workability, have been extensively used in applications such as automobiles (Japanese Unexamined Patent Publication (Kokai) No. 58-185752).
In order to further improve the workability, various studies have been made on the compositions of ultra low carbon steels and their production processes.
For example, Japanese Unexamined Patent Publications (Kokai) No. 3-130323, No. 4-143228, and No. 4-116124 disclose that excellent workability can be provided by minimizing the content of C, Mn, P and other elements in an ultra low carbon steel with Ti added thereto. In the inventions described therein, however, no mention is made of an improvement in the yield in the end portions in the widthwise direction and longitudinal direction of the steel strip (coil). Further, the techniques disclosed therein, unlike the technique according to the present invention, do not positively utilize Ti and Nb carbosulfides, Ti carbide and the like.
Japanese Unexamined Patent Publications (Kokai) No. 3-170618 and No. 4-52229 describe a reduction in a variation of properties of materials. According to the inventions described herein, however, the reduction ratio in finish hot rolling should be large, and, at the same time, an enhanced coiling temperature after the hot rolling is necessary, resulting in application of large load to the step of hot rolling.
The effect of the present invention can be attained also in P- or Si-strengthened high-strength cold rolled steel sheets possessing good workability. Representative techniques on these steel sheets are disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) Nos. 59-31827 and 59-38337, Japanese Examined Patent Publication (Kokoku) No. 57-57945, and Japanese Unexamined Patent Publication (Kokai) No. 61-276931. In these techniques, however, no device for improving the yield in the end portions in the widthwise direction and longitudinal direction of the coil is provided. Further, the techniques disclosed therein, unlike the technique according to the present invention, do not positively utilize Ti and Nb carbosulfides.
For ultra low carbon steels with Ti or a combination of Ti and Nb added thereto, it is common practice to coil a steel strip, after hot rolling, at an elevated temperature. According to this method, the coiling at an elevated temperature causes C to be precipitated as TiC or NbC, resulting in reduced C in solid solution, which in turn ensures good properties after cold rolling and annealing. Since, however, the end portions in the widthwise direction and the end portions in the longitudinal direction of hot rolled coils are very rapidly cooled during and after coiling, the precipitation of TiC and NbC is unsatisfactory, leading to deteriorated properties in these portions. For this reason, in fact, the end portions of hot rolled sheets or cold rolled sheets are, in many cases, cut off, increasing the production cost of the ultra low carbon steel.